CEREBROSPINAL FLUID (CSF) AND THE BLOOD-BRAIN BARRIER (BBB)

Textbook of Medical Physiology, Guyton and Hall, Ch. 61 Student Preparation Textbook of Medical Physiology, Guyton and Hall, Ch. 61

Cerebrospinal Fluid Cerebrospinal fluid (CSF) is a clear fluid present in the ventricles of the brain, the central canal of the spinal cord, and the subarachnoid space. Normal: almost no blood cells, little protein.

CSF Cont. Its rate of formation, flow, and absorption is sufficiently high to cause its replacement several times daily. Measuring its pressure, performing cell counts, and analyzing levels of various biochemical constituents is accomplished with a spinal tap.

CSF Cont. Injecting radiopaque dyes into the subarachnoid space is the basis of a common neuroradiologic technique called a myelogram. Obstruction of flow of CSF is a common cause of hydrocephalus.

CSF Cont. An understanding of the formation, flow, and absorption of CSF is essential to an understanding of these diagnostic procedures and the pathophysiology of hydrocephalus.

Cerebrospinal Fluid A clear, colorless fluid that surrounds and permeates the CNS. Offers support, protection and nourishment. Functions: Protection of cranial contents Modulates pressure changes (same specific gravity as brain) Serves as a chemical buffer to maintain constant ionic environment Serves as a transport medium for nutrients and metabolites, endocrine substances and even neurotransmitters

Location of CSF Two lateral ventricles Third ventricle Fourth ventricle Spinal cord central canal Subarachnoid space Continuous with extracellular fluid of brain parenchyma

Formation of CSF Choroid plexuses of lateral, third and fourth ventricles Ependymal lining of ventricular system Pia-glial membrane Blood vessels

Formation of CSF Cells are believed to actively secrete Na+ into the ventricular system in exchange for K+. Sodium ions electrically attract Cl- and osmotically draw water from the blood vascular system to constitute the CSF.

Composition of CSF

Circulation of CSF Due to pulsation of blood in choroid plexus Due to pulsation of ependymal cells Lateral ventricles interventricular foramen of Monroe third ventricle mesencephalic aqueduct (aqueduct of Sylvius) fourth ventricle spinal cord central canal; also, out the lateral apertures to the subarachnoid space to the venous system

Absorption of CSF Through the arachnoid villi, a protrusion of arachnoid membrane into the central venous sinus and other sinuses A valve opens when CSF pressure exceeds venous pressure Absorption by veins and capillaries of CNS

CSF Pressure Dog - 170 mm H2O Cow - 200 mm H2O Horse - 400 mm H2O Subarachnoid hemorrhage blocks absorption, raises pressure Congenital excessive formation or blockage to flow raises pressure Regulated by rate of absorption and rate of formation. Brain tumors lower absorption & raise pressure Inflamed meninges can cause excessive formation & raise pressure

Jugular compression maneuver (Queckenstedt maneuver) Compress jugular veins in neck - increases CSF pressure at the cerebellomedullary cistern and the lumbar cistern (normal) Decreased pressure at lumbar cistern indicates blockage of spinal central canal

Cerebrospinal fluid collection

Cerebrospinal fluid collection Small animals: atlanto-occipital (cerebellomedullary) cistern neck

Cerebrospinal fluid collection Large animals cerebellomedullary cistern Lumbar cistern (L5-L6)

CSF collection

Dog Lumbar Myelogram

Hydrocephalus (water on the brain) An abnormal increase in the volume of CSF Symptoms: depression, sleep, changes in behavior, spastic paresis, papilledema, bulging of skull in young, behavioral change, pacing, head pressing, seizures, cranial nerve deficits, vestibular signs

Hydrocephalus Communicating – due to excessive formation of CSF or lack of absorption Non-communicating – due to obstruction of flow through ventricle system

Ventriculoperitoneal Shunt

Coup – Coup-Contrecoup Closed Head Injuries With a blow to the back of the head, the forehead strikes the surface (coup), then the brain bounces back against rear skull (contrecoup) Coup Coup-contrecoup

Blood-Brain Barrier (BBB) Direct injection of large molecules (e.g., a dye) into the venous system will often penetrate all tissues except the brain. This observation led to the notion of a physiological barrier to flow of substances from blood to brain tissue – the blood brain barrier. The BBB functions to preserve a stable environment for neurons and glia of the CNS

Differences between brain and general capillaries General capillary small solutes can diffuse through intercellular clefts pinocytosis independent of molecular size (pass large molecules) fenestrated Brain capillary tight junctions reduced pinocytosis no fenestra astrocyte foot processes

General vs. Brain Capillaries

Brain capillary Tight junctions Surrounded by astrocytes Few vesicles Carrier mediated transport of glucose and amino acids

Blood-Brain Barrier Lipid soluble molecules will cross Negatively charged molecules cross more freely than positively charged ones High CO2/low O2 produce vasodilation and decrease resistance of BBB Injury or inflammation decreases resistance of BBB (allows some antibiotics to be used for treatment)

Blood-Brain Barrier (BBB) Selected brain areas are not protected by the BBB – the circumventricular organs (CVO) bordering on the 3rd and 4th ventricles parts of the hypothalamus - median eminence neurohypophysis pineal gland area postrema subfornical organ subcommissural organ

Blood – CSF Barrier Barrier from blood to the CSF Similar structural mechanism to the BBB